Artturi Virtanen (1952)

Atmospheric nitrogen as a sustainer of life on earth (German presentation)

Artturi Virtanen (1952)

Atmospheric nitrogen as a sustainer of life on earth (German presentation)

Comment

Virtanen was a Finish biochemist, being educated in physical chemistry, bacteriology and enzymology. In more than 50 years of research activity, he worked on problems connected to the dairy industry, agriculture and human nutrition. In the present talk he focuses on an issue which is still highly topical in the context of global nutrition: pathways of nitrogen uptake by plants. Nitrogen, phosphorous (via phosphate) and potassium (via potash) are the elements soils are commonly fertilized with to enhance crop yield. However, nitrogen stands out of this trio, because contrary to phosphorous and potassium, it is contained in the air we breathe, and not in a small quantity: the nitrogen in our entire atmosphere has a weight of approximately 4 quadrillion tons. At the same time, in 2010, approximately 135 million tonnes of nitrogen were industrially converted to fertilizers [1]. This nitrogen mostly comes from atmospheric, elementary nitrogen, which is converted to ammonia (NH3) by reaction with elementary hydrogen. This reaction is done by the so-called Haber-Bosch method, which is connected to a series of Nobel Prizes: Fritz Haber (Chemistry) in 1918 for the invention of the method. Carl Bosch and Friedrich Bergius (Chemistry) in 1931 for general technical improvements allowing an industrial scale-up and Gerhard Ertl (Chemistry) in 2007, inter alia for suggesting a complete theoretical explanation of the reaction mechanism. The Haber-Bosch method is still used today. However, it is so energy-hungry that it accounts for 1-2 % of the global energy consumption. If plants could obtain nitrogen directly from the air, a great deal of energy and money could be saved. Unfortunately, plants themselves are unable to comply: elementary nitrogen is very stable and difficult to metabolize – as opposed to the nitrogen contained in nitrate or ammonium salts, for instance. Still, by symbiotically associating to bacteria, certain plants, the legumes (including important crops like beans, peas, lentils, soy and peanuts), have found a way to access aerial nitrogen. In his talk, Virtanen describes experiments aimed at elucidating the extent and mechanism of this aerial nitrogen uptake. He begins by mentioning that such research is complicated by the fact that the enzymes involved in nitrogen fixation may not be studied outside living cells (in fact, this only became possible in 1960, 8 years after the talk). He then formulates what he believed to be the major question of the time with respect to nitrogen fixation: is the first step of its mechanism oxidative (leading to NO species) or reductive (leading to NH species)? After a review of the current knowledge he makes the “educated guess” that the first step of aerobic nitrogen fixation is oxidative, while the first step of anaerobic nitrogen fixation is reductive. Today we know that there is merely one, reductive way of aerial nitrogen fixation. However, other remarks of Virtanen on the character of the enzyme system, namely that it is likely to contain iron and molybdenum, have proven correct. In the last part of the talk, Virtanen mentions his discovery of leghemoglobin, a hemoglobin like macromolecule, which is synthesized by legumes as a reaction to the colonization of their roots by nitrogen fixing bacteria. He points out that the production of leghemoglobin is tightly associated with the plant’s ability to fix nitrogen. And in fact, the role of leghemoglobin in the symbiosis of plants and bacteria was later found to be the scavenging of oxygen, which would otherwise inhibit the enzymes responsible for the nitrogen fixation process. Eventually, Virtanen stresses the importance of introducing legumes to agricultural crop cycles, as they can significantly reduce the need for fertilization. In fact, Virtanen mentions good experiences he made with legumes on his own little seaside farm in Finland, which he had bought to test his research results in practice. In view of a steadily increasing human population, legumes could help to create better living conditions by ensuring food supply, he says. But he also mentions that the chemical industry naturally has a high interest in selling nitrogen fertilizers. Looking at the development of nitrogen fertilizer sales in the 60 years after Virtanen’s talk (they increased approximately ten-fold), it sure seems as if Virtanen’s suggestions of 1952 would have deserved more attention.

David Siegel

[1] Food and Agriculture Organization of the United Nations (FAO), Current world fertilizer trends and outlook, 2010.